To help appreciate the significant advances made by the present invention in the field of dispensing and, in particular, the field of foam dispensing, provided below is a discussion of the prior art efforts to provide a dispensing system for use in dispensing a foam such as a polyurethane foam.
FIG. 1 of the present invention schematically illustrates the general set up of a dispensing system for mixing and dispensing two liquids (liquids A and B) such as organic resins and polyisocyanates which react to form a polyurethane foam. As shown in FIG. 1, a chemical "A" supply means 1100 is provided for feeding to the dispensing apparatus 1102 a first chemical such as a liquid organic resin, and a chemical "B" supply means 1104 is provided for feeding to dispensing apparatus 1102 a second chemical such as a polyisocyanate. The organic resin and polyisocyanate are mixed within a mixing chamber of dispensing apparatus 1102 to form polyurethane foam which is discharged from dispensing apparatus 1102.
One particular problem with the dispensing of polyurethane foam of this type is that the organic resin and polyisocyanate react rapidly, and can accumulate over the external and internal surfaces of the dispensing apparatus. Under some mixes, an expansion of about 200 times in volume takes place and the mixed chemicals become very adhesive in nature. Thus, preventing the foam mix from reaching undesirable areas such as the chemical introduction ports and the discharge outlet is difficult. Also, once the foam material reaches a location and cures, it is difficult to remove the residue left as the residue strongly adheres to the underlying surface. A build up of foam residue can result in, for example, the binding, sticking or locking up of components, as well as degradation of the contacting surfaces, particularly when, a plastic, cold flow material such as TEFLON.RTM. material is used in the mixing chamber region. The sticking of components due to the build up of foam precursor material also requires an increase in the driving motor output used to reciprocate purge or valve rods associated with many prior art designs. Accordingly, in anticipation of this inevitable build up and increase in motive requirements such dispensers is sized to accommodate this build up in motive force requirement.
The inevitable build up of cured foam in the dispenser over time also results in partial or complete blockage of the chemical inlet ports and passageways which can lead to a change in flow characteristics with respect to chemicals A and B and therefore a change in the chemical A and B input ratio. Even a minor obstruction in an inlet port passageway prevents the output of a precise and consistent amount of foam over time, which, among other problems, presents a significant quality control problem with respect to the resultant foam product. That is, the poor ratio associated with prior art dispensers having chemical inlet or inlets at least partially blocked with cured foam build-up leads to an off-ratio mix and a resultant product not having high quality characteristics. Also, as the amount of build up, and thus the degree of obstruction, varies over time, there is a corresponding variation in the amount of chemical precursor introduced into the mixing chamber and outputted from the dispenser. This, coupled with the change in foam development brought about by the off-ratio foam introduction, leads to significant changes in the amount of foam produced from output to output over time and the aforementioned quality control problems.
The partial or complete blockage of the chemical inlet ports and binding or sticking of components in the dispenser are just some examples of the dispenser's performance being degraded by the build up of foam residue on a surface thereof. Actually, starting from the time of first use, the dispensing operation tends to deteriorate such that frequent dispensing apparatus servicing (e.g., disassembly and cleaning or replacement) is required. For high use operators of the prior art foam dispensers, servicing often takes place on a daily basis, while for even less high volume producers, weekly servicing is not uncommon. Of course, the down time associated with such servicing requirements, as well as the costs associated with such servicing is particularly problematic in the industry. Prior to servicing, however, the inevitable deterioration in operation typically results in a substantial decrease in foam output of the gun for reasons such as those outlined above. A not atypical scenario involves a dispenser dropping from a first cycle output flow rate of 8 to 10 lb./min. mixed foam precursor down to 4 or 5 lb./min. often within a relatively short time frame (e.g., less than 20,000 cycles of use). After running at this reduced capacity, the tenacious nature of the mixed foam precursor material inevitably leads to a complete locking up of the gun or a dispensing ratio that is totally unacceptable.
The prior art, in an effort to prolong the time between servicing, has relied upon complicated systems in an effort to maintain the optimum chemical A and chemical B input ratio despite the inevitable partial and often unequal obstruction of the inlet ports for the chemicals leading into the mixing chamber. For example, complicated pumping and sensing systems have been developed to vary the pumping characteristics of one or the other chemical supply pumps to account for a greater degree of blockage in one port versus the other. In addition to the high costs associated with these complicated prior art pumping and associated control systems, the added complexity can lead to increased chances of a breakdown as well as more difficult operation and higher skilled personnel to monitor the dispensing system. Moreover, this additional equipment can do nothing to help in the internal build up of foam in the dispenser which causes the increased sticking in reciprocating components and the eventual lock up of the dispenser.
FIGS. 2 and 4 illustrate two different prior art dispensing apparatuses which are representative of those being used in this field. The structure and operation of these systems are described below and in further detail in the detailed disclosure portion of the present application to help illustrate the significant advances made by the present invention over the prior art. These dispensing apparatuses are disclosed in U.S. Pat. Nos. 5,211,311 and 3,945,569, respectively, and both of these patents are incorporated herein by reference. Also, reference is made to U.S. Pat. Nos. 4,568,003; 4,469,251; 4,898,327; and 5,090,814 as further illustrating the state of the art and these patents are also incorporated herein by reference. The embodiment shown in FIGS. 2 and 3 is representative of a dispensing apparatus which does not involve the use of a solvent, features a purge rod (as opposed to a valving rod discussed below for the FIG. 4 embodiment) and features valves positioned to allow or disallow the flow of fluid to the mixing chamber through the mixing chamber inlet ports. Accordingly, the FIG. 1 schematic shows the optional inclusion of solvent supply means 1106 by a dashed line representation.
The FIG. 4 system is representative of a dispensing apparatus which uses a reciprocating valving rod to allow for and to disallow the introduction of the chemical precursors into the mixing chamber of the dispensing apparatus. In addition, a pool of solvent is provided through which an intermediate portion of the valving rod reciprocates.
The purge rod and the valving rod in the two above described prior art embodiments are placed in an interference fit or zero tolerance relationship with respect to the mixing chamber's wall encompassing the reciprocating rod. The reason for this is that the dispensing apparatuses are designed to preclude any sort of leakage past the rods and to prevent the possibility of foam material remaining anywhere in the mixing chamber long enough to harden and form an adhered deposit. In an effort to further assure against the hardening of a deposit, solvent utilizing systems such as that shown in FIG. 4 include a recessed area in the interference valving rod which traps a quantity of fluid rearward of its front end to bring the solvent into a particular portion of the mixing chamber prior to the rod being reciprocated back to a dispensing mode position.
As the prior art dispensing apparatuses rely upon a valving rod in an interference fit relationship with the wall defining the mixing chamber or a non-valving purge rod in a similar interference fit relationship, the rod in each case has a relatively high friction coefficient and the motive force required to reciprocate the rod is relatively high. The motive force relied upon in the prior art has typically been a piston/pressurized air (e.g., 100 psi) assembly with the rod being attached at its rear end to the reciprocating piston. An air piston force of 1,000 lbs. is typically involved in moving a piston travelling in a back chamber of the dispenser and a liquid tight seal with respect to the piston chamber.
This relatively high motive force is also present in prior art systems such as that disclosed in U.S. Pat. No. 4,568,003, wherein the reciprocating rod is in an interference relationship with a core or block of Teflon material having a through-hole within which the valving rod reciprocates. Despite the relatively low friction coefficient of Teflon material, the cold flowing core is placed in a high pressure state by enclosing the core in an encompassing housing with a retaining plate held under a relatively large biasing force by a series of Bellville washers. This biasing arrangement therefor maintains, by way of cold flow of the Teflon block, an interference fit between the reciprocating valving rod and surrounding block of Teflon material defining the mixing chamber despite a scraping or wearing away of the Teflon block.
Because of the motive forces involved, the prior art systems such as those described above virtually all have a mixing chamber of about 3/16 of an inch (4.75 mm) in diameter which is generally considered the upper end for most practical applications of the prior art dispensing apparatuses. This is because any attempt to increase the diameter of the mixing chamber leads to a substantial increase in the surface area contact between the reciprocating rod and mixing chamber wall and thus a corresponding substantial increase in the motive force required for reciprocation of the rod. This restriction imposed on the diameter of the mixing chamber presents a serious limitation on the throughput volume of foam that can be dispensed. For example, any attempt to modify a gun such as that shown in FIG. 4 to have a larger discharge outlet would require a significant increase in size in the motive force providing means (particularly in view of the fact that the motive force providing means needs to be built not only large enough to overcome the substantial increase in resistance created by the increased surface area contact between the purging of valving rod with the mixing chamber wall, but also to compensate for the sticking problem discussed above) and housing and a corresponding increase in the weight of the dispenser and housing as to render the design unpractical for many uses, particularly one involving hand held dispensing. FIGS. 2 and 4 are representative of the hand-held or gun dispensing apparatuses used in the art. Examples of non-gun or support structure mounted dispensing apparatus can be seen, for example, in U.S. Pat. Nos. 4,426,023; 4,390,337; and 5,255,847, which are also incorporated herein by reference.
A typical throughput for a prior art system is, in its first few cycles, 8 to 10 lbs./min. on average for a 3/16 inch discharge outlet which produces an exit velocity in the foam being dispensed which is considered the maximum allowable as any increase in velocity produces unacceptable backsplash and splattering. Also, because of the small amount of time between the moment the chemicals A and B make contact and the moment foaming is complete, and even the smaller window of opportunity within which the foam is in a workable state, operators often find it difficult to properly mold the foam (e.g., a bag of foam) about the configuration of an object to be protected. Once the two foam precursors come in contact there is a period within which the mix is in a creamy, not appreciably rising, state. This is short lived, however, because soon the mix begins expanding at an accelerated rate from the cream state to a solid foam body. From the standpoint of conforming a protective foam body about an object to be protected, it is preferable to place the foam while mainly in a cream state within a mold extending about the object to be protected. With many foam mixtures this would be a 6 to 8 second window. However, under the prior art systems, it often takes much of, if not all, that window just to dispense the foam (e.g., a one pound quantity of foam) from the dispenser. If any additional steps are involved, such as removing and positioning a bag of foam in a container containing a product to be protected, the window of opportunity is surpassed. This is especially true with regard to prior art dispensers which are in between an off-the-shelf state and lock up as such dispensers operate at reduced output capacity and thus take a longer period of time to dispense a given quantity of foam material. This problem of low volume dispensing within a given time frame is due largely to the 3/16 inch discharge outlet threshold faced by the prior art systems. While an increase in the velocity of the mixture being dispensed can lessen the time frame between the time the discharge is initiated and terminated for a given volume of foam mix, an increase in velocity leads to the aforementioned problem of an unacceptable amount of backsplash and splatter.
The backsplash of the prior art system is due in part to the high output velocity (e.g., 80 in/sec.) associated with the relatively small passageway diameter of the mixing chamber's output. Backsplash or splatter of reacted chemistry is thus a typical detriment found in prior art dispensing systems because of the above noted diameter, throughput, velocity relationship and limitations. Because reacted chemistry must be effectively directed into the receiving chamber in order to obtain the most efficient associated labor and material performance, a larger diameter, higher throughput, lower velocity relationship is preferable for optimal functionality of the dispensing system. However, because of the above described reasons, the prior art systems have been unable to provide such a dispensing system.
The inability to increase foam output within a given period of time also leads to a decrease in the overall yield of foam produced. As the initially discharged foam begins to expand, the later discharged foam comes in contact with it. This results in a decrease in foam yield as the contact with the subsequently discharged foam prevents optimum expansion of the rising foam mixture.
Another problem which is prevalent in many prior art dispensing systems is the problem of chemical precursor cross-over, while the rod is in parked state. Cross-over occurs due to leakage of chemical precursors around the rod (particularly a valve rod) when in its parked state. As the prior art systems are not designed to allow for flow of chemical precursors between the rod and mixing chamber housing, the cross-over leads to significant problems. If the cross-over does not lead to an immediate locking up of the dispenser, it can lead to further degradation of the system until it either locks up or is otherwise non-functional. Many efforts have been made in the prior art to avoid this problem with U.S. Pat. No. 4,159,079 being representative.
As discussed above, a system using a valving rod, such as the one shown in the prior art dispenser of FIG. 4, is particularly susceptible to premature chemical contact due to the cross-over of the chemicals through leakage between the valve rod and mixing chamber. This problem is lessened in the prior art system shown in FIGS. 2 and 3 which has a separate upstream valve assembly and purge rod in place of a valve rod. However, while lessening of the crossover leakage problem, the upstream positioned valving arrangement presents another problem in that there is added a requirement for synchronizing the opening and closing of the upstream chemical passageway ports for chemicals A and B. For example, because the needle valves in the FIG. 3 prior art depiction are in line with the chemical ports, wear in the seals can lead to one or both sides leading. To compensate for this possibility, the extension of the needle valves can be adjusted in an effort to maintain a constant seal pressure contact for both chemical ports. To provide for synchronized opening and closing of both ports despite variations over time in the dispenser's features (e.g., seal wear), adjustable yoke assembly 837 is provided in the FIG. 2 and 3 embodiment. The fine tuning of the yoke assembly required by an operator to achieve synchronized chemical inlet opening and closing represents an additional problem in prior art dispensing devices.
Another particularly problematic area in these prior art dispensing apparatuses is found at the outlet tip of the dispensing apparatus. Whether due to backsplash, internal leakage or leftover deposits, there is a tendency for the prior art dispensing apparatus to have the valving or purge rod lock up at the end portion of the dispensing apparatus or to have the dispenser's port become blocked. Various methods have been developed in the art to avoid this problem such as a grinding of the tip (requires a hardening of the material forming the tip) or applying solvent at the tip through a separate line and solvent supply assembly joined to the tip. An example of providing a solvent to the tip of a dispensing apparatus is found in U.S. Pat. No. 4,426,003, which features a cap-like structure with multiple solvent ports mounted in the tip region of the dispensing apparatus and U.S. Pat. No. 4,898,327, which features a porous tip permeable to solvent.
The prior art dispensing apparatuses are also difficult to service due, for example, to the complexity of their designs, the large number of components involved, the deterioration of components, sticking and jamming, the high force needed to remove the purge or valve rod, and difficulty in accessing the various components. For example, even with the appropriate tools, it is often very difficult to separate the rod from the mixing chamber due to the adhesive quality of the foam precursor build up. Also, the chemical inlet ports are very difficult to clean due to their small diameter (drilling out of deposits often being involved). The cleaning of the ports can also lead to changes between the diameter of the chemical A and B ports and a resultant off-ratio mix. The prior art dispensing apparatuses also often have a relatively short halflife of 20,000 cycles or so between servicing or replacement requirements. The problems associated with the prior art and the difficulty involved in servicing such systems often results in disposal rather than a continued effort to keep using the prior art dispensing apparatuses.